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Adult male Western Bluebird, Baja California, December

Western Bluebirds are small thrushes that breed throughout much of the western United States, Mexico, and southwestern Canada. Males have brilliant blue plumage on their heads, wings, and tails, rust colored breasts, and, frequently, chestnut back patches; females are duller and have more brown and gray in their feathers. Insectivorous during the warmer months, individuals forage primarily on berries and fruits through the winter; wintering individuals are especially abundant in years and in areas when mistletoe (Phoradendron spp.) and juniper (Juniperus spp.) berry crops are plentiful.

Western Bluebirds are socially monogamous. Both parents usually care for young, but they also seek matings outside the pair bond, with the result that offspring are not always related to the attendant male. Helpers at the nest occur in some populations and are frequently adult male relatives, often putative sons of the resident pair, or juveniles from earlier broods.

Western Bluebirds can usually be found in open, parklike forests, edge habitats, burned areas and where moderate amounts of logging have occurred, provided a sufficient number of larger trees and snags remain to provide nest sites and perches. Unlike Eastern (Sialia sialis) and Mountain (Sialia currucoides) bluebirds, Western Bluebirds do not favor large, open meadows. Clear-cutting, snag removal, fire suppression, and any changes in land use that cause open forest and edge habitat to be diminished adversely affect Western Bluebird populations.

Apparent declines in numbers of this species in the Pacific Northwest and British Columbia, especially in regions west of the Cascade Range, have generated concern. In response, bluebird enthusiasts in Washington, Oregon, and British Columbia have established trails of nest boxes in an effort to reestablish local breeding populations.